Select Multiple Key Frames in Timeline Editor
The timeline isn't actually an editor. It's simply a viewing port for all the keyframes in your animation. What you can do is:


Go to the Graph editor and select the objects that are keyframed
Move the playhead in the timeline to the keyframe you wish to delete
Hover your cursor over the graph editor and press Ctrl-K
Go to the next keyframe and repeat steps 2-3
Press X to delete the selected keyframes


PLEASE NOTE This process works if you want to delete all keyframed information about the selected objects. If, say, you just want to delete X-Location info but not Y-Location info, you have to manually select each keyframe in the Graph Editor.

Hope this helps!